SRAM cell structure and circuits

ABSTRACT

An SRAM circuit structure and method for reducing leakage currents and/or increasing the speed of the devices. Various forms of SRAM devices may be fabricated utilizing the techniques, such as single port and dual port RAM devices. By way of example the SRAM structure utilizes separate write and read lines, splits the circuit into portions which can benefit from having differing threshold levels, and can allow splitting read path transistors for connection to a first terminal and a virtual node connected to a source transistor. The structure is particularly well suited for forming transistors in a combination of NMOS and PMOS, or solely in NMOS. Memory arrays may be organized according to the invention in a number of different distributed or lumped arrangements with the reference read paths and sense blocks being either shared or dedicated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/883,581, filed on Jun. 30, 2004, now U.S. Pat. No. ______,incorporated herein by reference in its entirety, which claims priorityfrom U.S. provisional application Ser. No. 60/484,565 filed on Jul. 1,2003, incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION BY REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 114.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains generally to semiconductor memory, and moreparticularly to static random access memory circuits.

2. Description of Related Art

Static Random Access Memory (SRAM) is a form of electronic data storagewhich retains data as long as power is supplied. Static RAMs are widelyutilized within all manner of electronic devices, and are particularlywell-suited for use in portable or hand-held applications, as well as inhigh performance device applications. In portable or hand-held deviceapplications, such as cell phones, SRAMs provide stable data retentionwithout support circuits, thus keeping complexity low while providingrobust data retention. In high performance applications, such asmicroprocessor caching, since the SRAM can provide fast access timeswhile not requiring the cell data refresh operations required in DynamicRandom Access Memory (DRAM).

FIG. 1 shows a typical SRAM cell consisting of six transistors (6T SRAM)and related peripheral circuits. For example, when node C_j isprecharged at Vdd (data H), mp2_j is turned off and mn2_j is turned on.Node CBj is set Vss (data L). Therefore, mp1_j is turned on and mn1_j isturned off. Hence, as long as power is supplied, data at C and CB aremaintained high and low respectively.

FIG. 2 shows a read timing diagram of the conventional 6T SRAM cellshown in FIG. 1. In a precharge cycle, PPREi is at logic low and mpp1_iand mpp2_i are turned on. So, bit line pairs (BL_i and BLB_i) areprecharged at V_(dd), logic high. When a word line (WL1_i) is enabled, abit line is discharged depending on the stored data. For example, nodeC_j is high and CB_j is low.

According to the word line enabling, mn3_j and mn4_j are turned on.Since CB_j is low and mn2_j is turned on, the voltage of BLBi isdischarged slowly through mn4_j and mn2_j. When a certain amount ofvoltage difference between bit line pairs arises, a sensing enablesignal (PSAEi) is enabled to amplify the signal difference. The voltagedifference on the bit line pairs is amplified by the sense amplifier (i)and a full CMOS output pairs (Di and Dbi) are generated at the outputsof the sense amplifier.

Since the typical 6T SRAM cell creates a signal difference on bit linesby itself, the read speed of SRAM is faster than that of DRAM, in whicha charge sharing time between the bit line and cell capacitances isneeded and read speed is slowed. This 6T SRAM cell has a very stablestructure and is widely used in typical SRAM design. However, there is atrade-off between power consumption and read speed. As the minimumfeature size (i.e. design rule) decreases and the threshold voltage oftransistors is reduced to maintain performance as operating voltage islowered, the leakage current (i.e. standby current) becomes an importantfactor. In this example, since C_j is at a high level and CB_j is at alow level, mp2_j and mn1_j are turned off. Even though these twotransistors are in an off state, there is a current flowing through thedevices referred to as a cell leakage current. At 0.18 μm technology,this leakage current is on the order of ƒA (10e-15) and can besubstantially ignored for most applications.

However, with regard to more advanced technology such as 0.13 μmtechnology, since this current is then on the order of tens of nA(10e-9), the level of current can no longer be ignored. For example, fora 16 Mb SRAM, when a cell leakage current is about 10 nA, the totalcurrent is 16*1024*1024*10*1 e-9=16 mA. This level of leakage currentequates to a large portion of the total power consumption for thedevice. It should also be appreciated that this leakage current istemperature dependent, increasing in response increasing temperature.For more advanced technology such as 0.11 μm technology, the cellleakage current increases significantly. Therefore, the powerconsumption component which arises as a result of cell leakage currentbecomes quite substantial. As the systems relying on SRAM becomeincreasingly complex, the density of SRAM will continue to increase, andthe total power consumed by cell leakage currents based on conventionalSRAM architectures will continue to increase.

As mentioned earlier, there is a trade-off to be made between powerconsumption and cell read speed. Since the cell read speed is determinedby how fast a bit line node (e.g., CB_j) is discharged through the cellpull-down transistor (mn1_j or mn_j). Therefore, the sizes of the cellaccess transistor (mn3_j or mn4_j) and the cell pull-down transistor(mn1_j or mn2_j) need to increase to enhance the read speed. However,when these cell access transistors and cell pull-down transistorsincrease in size, leakage currents flowing through these transistorsalso increase. In this example, when these transistors increase in size,leakage current flowing through a pair of mn4_j and mn2_j and mn1_jincreases. Therefore, a trade-off between the cell leakage current andthe cell read time makes SRAM design complicated and difficult as theoperating voltage goes down.

In general, two classes of SRAM cells are implemented depending on thewhether the SRAM is used with a low power or high performanceapplication. With regard to low power applications, such as low powerhand-held devices, the stand-by current (i.e. power consumption whilethe chip is in a stand-by mode), is often the most importantconsideration as these low power portable applications often rely onbattery operation wherein stand-by current is a major determiner ofbattery life. This is in contrast to high performance applications suchas cache memory, wherein cell data read speed is of critical importance.However, due to a drastic increase of the cell leakage current, theconventional 6T SRAM cell structure is facing a technical barrier tomeet the design requirement. When device sizes increase and thethreshold voltage of transistors decreases to meet the required speed,the power consumption due to the cell leakage current is a concern. Whendevice sizes are scaled down and the threshold voltage of transistorsincreases to suppress leakage current, the cell read speed is degradeddue to reduced current driving capability of the cell access andpull-down transistors.

Accordingly, a need exists for advanced SRAM circuits and methods forreducing leakage currents without sacrificing read speed. The presentinvention fulfills that need and others, while overcoming the drawbacksfound in conventional SRAM architectures.

BRIEF SUMMARY OF THE INVENTION

A static random access memory (SRAM) circuit is described which providesreduced leakage currents and high reading speed. The novel architecturedescribed is configured with a novel read sensing structure that can beutilized in combination with differing voltage thresholds for differentfunctional blocks within the device. These aspects of the invention canbe utilized separately or in combinations to increase the speed ofmemory and/or to lower the power dissipation, such as resulting from theleakage currents.

One embodiment of the invention can be described as a memory deviceproviding static random-access memory, comprising: (a) a static memorycell structure having a plurality of data latches; (b) a plurality offunctional blocks within the cell structure including read, write andstorage. By configuring at least one of the functional blocks, such asthe read word circuit and/or the write word circuit, for a lowerthreshold voltage the leakage current for the device can besubstantially reduced. In one embodiment the read word signal and writeword signal are separated. An embodiment is described in which data isread through read transistors activated by the read lines whose outputsdrive alternating bit lines on successive words. A sense amplifierprovides differential sensing of the bit lines to drive the output data.

Another embodiment of the invention can be described as a memory devicehaving a static random-access memory configuration, comprising: (a) aplurality of static memory cells, such as formed from data latches; (b)a plurality of word lines for the read path; (c) a plurality of wordlines for the write path; (d) a plurality of functional blocks includingread, write and storage.

At least one of the functional blocks is preferably configured to havedifferent voltage threshold conditions than the other functional blocks.In one embodiment a reference path circuit provides a virtual node towhich read path transistors are connected between different bit lines,wherein the read lines are sensed using differential sensing by a senseamplifier, such as for suppressing leakage current of cell blocks.

Another embodiment of the invention can be described as a semiconductormemory circuit comprising: (a) a plurality of static memory storagefunctional blocks (cells) having a plurality of data latches configuredfor being read and written; (b) a memory cell reading transistorfunctional block having an input coupled to each storage functionalblock and an output coupled to one of at least two bit lines; and (c) asense amplifier coupled between the bit lines for detecting data beingread from said data latches in response to differential sensing betweensaid bit lines. Furthermore, the read and write functional blocks can beconfigured with lower voltage thresholds than the memory latchtransistors, while in another variation the voltage threshold of theread transistors is designed to have a lower threshold than the writetransistors.

The invention may also be described in terms of a method of accessingcells of a static memory, comprising: (a) maintaining data written totransistors of a first voltage threshold level forming a data latch fora static memory cell;

(b) applying a read word signal to activate read word transistors of asecond voltage threshold which is less than first voltage threshold; and(c) sensing the output of the read word transistors in a sense amplifiercoupled to a bit line to generate a data bit output.

The invention may also be described in terms of a method of accessingcells of a static memory, comprising: (a) maintaining data written totransistors forming a data latch for a static memory cell; (b) applyinga read word signals to activate read word transistors; and (c) sensingthe output of the read word transistors in a sense amplifier coupledbetween bit lines to generate a data bit output. Furthermore, functionalblocks within the static memory can be configured with lower voltagethresholds, such as the read block, or the read and write blocks,wherein faster speeds and lower leakage currents can be exhibited by thecircuit.

A number of aspects are described for the present invention, includingbut not limited to the following.

An aspect of the invention is an SRAM cell structure which has aplurality of functional blocks, the functional blocks including read,write and storage, wherein each functional block can have differentthreshold voltages.

Another aspect of the invention is an SRAM cell in which the read pathhas a lower threshold voltage than that of storage and write path.

Another aspect of the invention is an SRAM cell in which the read pathhas the lowest design threshold, the write path has a medium designthreshold and the storage path has the highest design threshold voltage.

Another aspect of the invention is an SRAM cell structure in which oneterminal of read path transistors are connected together with oneterminal of alternating read path transistors and to a virtual nodewhich is connected to a source transistor to suppress leakage current ofcell blocks.

Another aspect of the invention is an SRAM structure which has separatefunctional blocks and independent word lines for read and write paths.

Another aspect of the invention is an SRAM structure which has separatefunctional blocks and the same word line for read and write paths.

Another aspect of the invention is a reference read path scheme with aPMOS source transistor.

Another aspect of the invention is a reference read path scheme with anNMOS source transistor.

Another aspect of the invention is the architecture for the placement ofreference read paths (RRPs) and sense amplifiers (SAs) in distributed,lumped, or mixed configurations.

Another aspect of the invention is the architecture for the utilizationof either shared or dedicated reference read paths (RRPs) and senseamplifiers (SAs).

Another aspect of the invention is an ability to implement various formsof SRAM memory such as multi-port SRAM, embedded forms of SRAM, and soforth according to the teachings of the present invention.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic of a conventional 6T SRAM cell and peripheralcircuits.

FIG. 2 is a timing diagram for the conventional 6T SRAM cell of FIG. 1.

FIG. 3 is a schematic of an SRAM cell structure according to an aspectof the present invention, shown with sensing circuitry and circuits forcontrolling cell leakage current.

FIG. 4A-4D are timing diagrams for the SRAM cell shown in FIG. 3.

FIG. 5A-5F are block diagrams of SRAM memory organizations according toembodiments of the present invention, showing the placement of referenceand read paths.

FIG. 6 is a schematic of a dual-port SRAM cell structure according to anaspect of the present invention, showing sensing circuitry and circuitsfor controlling cell leakage current.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus generally shown inFIG. 3 through FIG. 6. It will be appreciated that the apparatus mayvary as to configuration and as to details of the parts, and that themethod may vary as to the specific steps and sequence, without departingfrom the basic concepts as disclosed herein.

The new SRAM cell structure and related sensing scheme are configuredfor overcoming a number of the problems which arise with conventionalSRAM architectures. One improvement is an architecture in which specificfunctional blocks can be designed with different threshold voltages tocontrol leakage. For example, the transistors of the read circuit beingconfigured with a lower voltage threshold than the storage transistors.

As a second example, the write circuit is configured with a lowervoltage threshold than the storage transistors, and the transistors ofthe read circuit are configured with a lower threshold than the writecircuit transistors. It will be appreciated that the inaccuracies infabrication lead to small differences in threshold voltages betweendevices, however, these slight random variations are not what is beingaddressed herein. In the present invention, the differences in thresholdvoltages are in response to the design of the transistors beingfabricated, and the difference in threshold is preferably over aboutfive to ten percent.

FIG. 3 illustrates an example embodiment 10 of a new SRAM cell structureand related sensing scheme according to the present invention. The newcell structure can be utilized within any size of memory block (i.e.128WL×256BL). The new structure comprises storage cells 12 (datalatches), a reference read path 14 and a sense amplifier 16. Unlike theconventional six transistor SRAM cell structure shown in FIG. 1, the newSRAM cell shown in FIG. 3 comprises eight transistors (mpa, mpb,mna−mnf. Six transistors (mpa, mpb and mna−mnd) are used to store thecell data and a write path to change the cell data while two transistors(mne, mnf are used for the cell read operation. The source of the cellread transistors (mne, mnf are connected together with that of adjacentcell read transistors and are linked to a sense amplifier. The sourcenode of all transistors in the entire memory block or part of the entirememory block can be connected together depending on different designtargets.

In this embodiment the logic threshold voltage levels are shown withstorage cell 18 having a normal or high voltage threshold, for the givenoperating voltage, while the cell read transistor section 20 isconfigured with a lower voltage threshold.

In a precharge state, when WWLi (write word line) and RWLi (read wordline) are at logic low, mnc, mnd and mne are turned off. When C1 is at ahigh level and CIB is low, mpb and mna are turned off and mpa and mnbare turned on. Thereby, the data C1 and C1B are maintained. Since RWL1is low, mne is off and there is no current path through mne and mnf froma bit line (BLBi) even though this bit line is precharged at V_(dd)which is similar to that of FIG. 1.

One of the differences between the embodied cell structure andconventional cell structures is the use of separate paths for read andwrite operations. In the conventional structure, the cell accesstransistor and the pull-down transistor need to be large for fast readand write operation. However, in that case the cell leakage currentbecomes a concern. In the new cell structure of the present invention,since the read (mne and mnf) and write paths (mnc and mnd) arepreferably separate and the storage portion of the circuit (mpa, mpb,mna and mnb) are isolated from the read path, a fast read speed isachieved along with a significantly suppressed cell leakage current.

In the conventional cell structure, the cell transistors cannot bereadily configured to support different threshold voltages due to atrade-off between leakage current and the cell read speed. When the celltransistors have a high threshold voltage to suppress the cell leakagecurrent, the cell read speed is degraded due to the reduced currentdriving capability of transistors, especially the cell access andpull-down transistors. However, in the new cell structure of the presentinvention, the threshold voltage of transistors can be controlled moreadaptively. For example, the storage block transistors (mpa, mpb, mnaand mnb) and the cell access transistors (mnc and mnd) are preferablyconfigured as high threshold voltage elements to suppress the leakagecurrent since these parts are not related with the cell read operation.Instead, the cell read transistors (mne and mnf) are configured to havelower threshold voltages to improve cell read speed.

In the new cell structure of the present invention, each section of thecircuit can be configured with different threshold voltages to meetdesign requirements. By way of example, the threshold voltages of thestorage part (storage functional block) and the write path (writefunctional block) can be configured at higher threshold levels than thatof the cell read path (read functional block). By establishing thedifferent threshold levels the cell leakage current through theturned-off transistors of the storage part and the cell accesstransistors from the bit lines can be suppressed significantly while thecell read speed can be improved. It should be appreciated that theleakage current of the transistor is reduced exponentially in responseto increasing the threshold voltage of the transistor. Therefore, thecircuit can utilize different threshold voltages, such as for thestorage part and the read and write access transistors, to arrive at arange of desired characteristics for the SRAM device. The SRAM cellstructures depicted in FIG. 3 having three separate functional blocks,storage, write and read paths. Due to the design of the present SRAMeach of these functional blocks can be designed with transistors havingdifferent threshold voltages depending on the design requirements.

Another important aspect of the invention is a sensing scheme relatedwith the cell structure explained above. In the inventive cell scheme,the read path is connected to one of the bit lines. For example, in thetop cell, the read transistors are connected to the bit line bar (BLBi)and for the next cell below, the read transistors are connected to thebit line (BLi). Therefore, the sensing scheme to detect the signaldifference on the bit line pairs is also important.

The bit line sensing scheme can realize two important designrequirements, suppressing leakage current through the read transistorsand increasing the speed by which signal differences on the bit line aresensed. It should be noted that the common signal BLi is preferablyplaced only in the submemory block or connected to the entire block.Sense amplifier 16 may be placed in respect to the bit line or sharedwith multiple bit lines.

A principal objective of the present invention is to suppress leakagecurrent in the storage portion and the write path by using smallertransistors with higher threshold voltages than utilized in conventionalSRAM cell structures. A separate read path is also preferably utilizedwhich has a lower voltage threshold than utilized in the storage cell.There is another current path in the new cell, which is related with theread transistors. When the bit line pairs are precharged to V_(dd) as inthe conventional cell structure shown in FIG. 1, even though one of theread transistors is off since the read word line (RWLi) is low, there isalso leakage current flowing through these transistors. For example, C1is set V_(dd), mnf is turned on but mne is turned off since RWLi is low.However, a leakage current through transistor mne flows even though thistransistor is turned off.

In one embodiment of the present invention, SRAM cell read speed isaided by increasing the size of the read transistors to about four timesthe conventional sizing. According to the invention, the readtransistors are configured with a lower threshold voltage to improve theread speed, the leakage current flowing through the read path is muchlarger than that flowing through other parts. Therefore, the new sensingscheme related with the new SRAM cell structure can suppress the leakagecurrent flowing through the read path.

One way to further suppress the leakage current is to configure the bitlines as floating in a precharge state, which results in eliminating thepower supply source from the bit line. That is, the bit line prechargetransistors (mppl_i and mpp2_i) are turned off during the prechargestate and then are activated prior to the read operation by setting thebit lines to V_(dd) as in the conventional scheme. Although thisstructure can suppress leakage current, a problem arises with so-called“bit-line hurting” to the cell, during the write operation. When thereis no power supply, the voltage of bit lines are virtually ground due tojunction leakage current. When the write word line (WWLi) is enabled,transistors mnc and mmd are turned on and node C1 and CBI are connectedto bit lines BLi and BLBi, respectively. Since the bit line capacitanceis substantially larger than that of cell transistors, around twentytimes larger, when the write transistors are switched on, node C1 andC1B are discharged due to a charge sharing effect until node C1 isrestored by transistor mpb. Since node C1 is high and node C1B is low,the degree of discharging is different. So, in the ideal case, eventhough node C1 and C1B is discharged due to charge sharing effect, nodeC1 is discharged less and due to signal difference on node C1 and nodeC1B, the voltage of C1 and C1B can be eventually restored to V_(dd) andV_(ss).

However, in the case of mismatches in transistor size and thresholdvoltages, a different situation can occur. For example, node C1 can bedischarged more or less, the stored data information can be changed,because the threshold voltage of transistor mpb is lower than that oftransistor mpa due to fabrication process variation. That is, data onnode C1 and node C1B can be changed from high and low to low and high,respectively. This is a possibility which can occur in the new cellstructure of the present invention.

There are additional things to consider regarding sensing operationswhen the bit line is floating. For instance, in order to eliminate amismatch in the voltage of the bit line pair, it is necessary to performan extra precharge operation to assure that invalid sensing does notarise. This extra operation represents a speed penalty as it delays theactual sensing process. The new cell structure of the present inventiondoes not require the use of this extra precharge operation.

The embodiment shown in the figure (FIG. 3) provides a novel sensingscheme which doesn't require the precharge operation mentioned above andwhich can suppress the leakage current flowing through the readtransistors.

The sense amplifier scheme which eliminates the precharge operation andsuppresses leakage current, can be implemented utilizing a new novelcircuit incorporating what is referred to herein as a reference readpath. The main idea of the reference read path is to provide a currentpath having current driving capability equal to about half that of thecell read transistors.

In fabricating the SRAM device embodiment shown in FIG. 3, the width ofeach cell read transistor is ‘W’ as shown by the area of cell readsection 20 surrounded by the dotted lines. Wherein two transistorshaving width of W are stacked and the drain of two stacked transistorsis connected to a bit line and the source of two stacked transistors isconnected to a virtual ground signal which is also connected to thesource of two stacked transistors in other SRAM cells.

In the top cell of FIG. 3, mne and mnf are two stacked transistorshaving width of ‘W’, wherein one terminal of transistor mne is connectedto BLBi and one terminal of transistor mnf is connected to a virtualsignal V_(g), shown represented as a dotted line interconnecting the twocell read sections with transistor mse(A). The other terminal of the twotransistors, mne and mnf, are connected with each other. The virtualsignal line is connected to a source transistor, mse, which is turned-onin read operations depending on input condition. In the reference readpath two transistors, (e.g., msa, msb, msc and msd) are stacked. Oneterminal of each pair of transistors is connected to each bit line. Forexample, the drain of transistor msa is connected to BLi, and the sourceof transistor msb is connected to V_(g), which is at the drain of sourcetransistor mse. The other two terminals of transistors msa and msb areconnected together. The gate of transistor msa is connected to referenceread word line (RRWLa) and the gate of transistor msb is connected to aread signal (RSi). Transistors msb, msc and msd are shown placedsimilarly to transistor msa. The drain of transistor msc is connected tobit line BLBi and the source of transistor msd is connected to thevirtual ground signal V_(g). The source of transistor msc and the drainof transistor msd are connected together. The gate of transistor msc isconnected to another reference read word line signal (RRWLb) and thegate of transistor msd is connected to the read signal (RSi). RRWLa andRRWLb are enabled selectively with address information, or enabled whenthe read path at the other line is enabled (i.e. RWLi is selected whenRRWLa is selected).

It should be noted that RRWLb and RRWLa are enabled selectively withaddress information and are enabled when the read path on the other lineis enabled (i.e. RWLi is selected when RRWLa is selected). The source ofthe source transistor mse is connected to the source of transistors msband msd, while its gate is connected to the read signal RSi and thedrain of source transistor mse is connected to a power source V_(ss).Note that the source transistor mse is a PMOS transistor in thisexample, however an NMOS transistor may be alternatively utilized. Theorder of stacked transistors, msa, msb, msc and msd, can be changedaccording to design implementation without departing from the invention.

The width of stacked transistors in the normal cell is ‘W’, but thewidth of a transistor (i.e. msa) is ‘W/2’. It means that current drivingcapability of stacked transistors in the reference read path is half ofstacked transistors in the normal cell. Actually, the current drivingcapability of stacked transistors is not exactly half of stackedtransistors in the normal cell but it is required to have smallercurrent driving capability of stacked transistors in the normal cell.The sizing of transistors in the reference read path is determinedaccording to the desired operation (i.e. half that of read pathtransistors).

FIG. 4A through FIG. 4D illustrate timing aspects of the new sensingscheme. FIG. 4A-4B depict sensing timing when RWLi is enabled. FIG. 4Adepicts the case when C1 is high and CIB is low, while FIG. 4B depictsC1 being low and CIB being high.

In a precharge cycle, bit line pairs are set to a voltage, typicallyV_(dd). When a word line (i.e. RWLI) is enabled the data of C1 is highand CIB is low (FIG. 4A) wherein transistor mnf is turned on. There is acurrent path established through transistor mne and mnf from BLBi. Thestacked transistors connected to BLi are selected in the cell that hasthe read transistors connected to BLBi. In other words signal RRWLa isenabled to turn on transistor msa. To read the cell data, read signalRSi is enabled. When WWLi, RRWLa and RSi are enabled, bit linesdischarge at different rates seen as the varying slopes of BLi and BLBi.Note that the width of transistor mnf is ‘W’ while that of transistormsa is ‘W/2’. Therefore, the discharging slope of BLBi is faster asshown in FIG. 4A than in FIG. 4B due to its larger transistor size andlarger current driving capability. Hence, the signal difference on thebit lines is developed when the cell is selected.

When the data of C1 is low, as a result of transistor mnf being turnedoff, there is no current path through stacked transistors from the bitline bar. Therefore, BLBi remains high and only the bit line, BLi, isdischarged through the reference current path, msa and msb. Hence asignal difference on the bit lines is developed.

FIG. 4C-4D depict sensing timing when RWLj is enabled. FIG. 4C depictsthe case when C2 is low and C2B is high, while FIG. 4D depicts C2 beinghigh and C2B being low.

When the other cell having the cell read transistors connected to theother bit line is enabled, the stacked reference current path isselected to develop a signal difference on the bit lines. For example,when RWLj is enabled and the data of C2 is low and C2B is high,transistor mnf is turned on and there is a current path throughtransistors mne and mnf. Since the cell having stacked transistors isconnected to BLi, the stacked transistors connected to BLBI areselected. That is, when RWLj is enabled, RRLWb is enabled and transistormsc is turned on. When the read signal, RSi, is enabled, a current paththrough transistors msc and msd from BLBi is formed to discharge BLBI.As explained earlier, due to smaller current driving capability ofstacked transistors in the reference path, the discharging slope of BLBiis slower than that of BLi and there is a signal difference on bitlines.

When the data of C2 is high and C2B is low, mnf′ is turned off and thereis no current path through mne′ and mnf′. Only the bit line bar, BLBi,is discharged through msc and msd. A signal difference is developed onthe bit line pairs.

Since the virtual ground signal V_(g) is connected to the sourcetransistor, the amount of leakage current of the cell is not the sum ofthe leakage current flowing through turned off read transistors of eachcell but is limited by the leakage current of the source transistor mse.

It will be appreciated that the above describes read word linesactivating transistors coupled to the bit lines for developing signaldifferences on the bit lines for detecting memory storage cell state.This sense amplifier scheme can suppress leakage current significantly.In this example, the use of a PMOS source transistor mse reduces leakagecurrent by making all transistors in the reference read pathreverse-biased. In the active mode, when RSi signal is enabled, thevoltage of V_(g) is discharged to V_(tp), where V_(tp) is the thresholdvoltage of the source PMOS transistor. When RSi goes to low in thestandby mode or when the related cell is not selected, such as whenRRWLa and RRWLb are low and RSi is also low, the gates of msa, msb, mscand msd are low and the gate of the PMOS source transistor mse goes tohigh. Since the voltage of V_(g) is V_(tp), V_(gs) of msb and msd is−V_(tp), which means that transistors msb and msd are reverse-biased.Since the gate voltage of the PMOS source transistor mse is V_(dd) andthe source voltage is V_(tp), V_(gs) of transistor mse is V_(dd)−V_(tp),which also means that the source transistor mse is reverse-biased. Sincethe voltage of V_(g) is V_(tp), the voltage of the source of transistormsa and the drain of transistor msb and the voltage of the source oftransistor msc and the drain of transistor msd are also positivevoltages.

Since the gate voltage of transistors msa and msc is low, V_(gs) oftransistors msa and msc is a negative voltage, which means that thesetwo transistors are reverse-biased. Even when one of the reference wordlines, such as RRWLa or RRWLb, is high when RSi is low, transistors msb,msd and mse are reverse-biased. Therefore, the leakage current flowingthrough transistors in the reference read path is suppressedsignificantly.

One issue related with the reference read path above is the use ofdifferent types of MOS transistors. One embodiment can be created withtransistors for carrying the reference current being NMOS transistors,while the source transistors are PMOS transistors. However, creating aPMOS transistor requires an NWELL structure which results in an areapenalty. To solve this problem, the PMOS source transistor can bereplaced by an NMOS transistor. In this case, all transistors in thereference read path have the same type, the area penalty due to theformation of NWELL for PMOS transistors can be minimized. In case of theNMOS source transistor, when the gate voltage is low to turn off thesource transistor, since the source of the NMOS source transistor isV_(ss), V_(gs) of the NMOS transistor is about zero volts instead of anegative voltage. Therefore, the leakage current flowing through thereference read path can be increased somewhat, however, it is still muchsmaller since the leakage current of the cell block is limited by thatof this NMOS source transistor.

A principal objective of the inventive sensing scheme is to have areference read path which can be selected alternatively, that is, acurrent path is formed from a bit line in the normal cell and areference current path is formed from the other bit line to develop asignal difference by different current driving of each current path.

When a signal difference is developed on the bit lines, the sensingenable signal SAE is enabled to amplify the signal difference. Areference read path is placed per a pair of bit lines or can be sharedby multiple bit line pairs. A sense amplifier is also positioned by apair of bit lines or can be shared by multiple bit line pairs. It shouldbe appreciated that the novel SRAM device can be implemented with anumber of sense amplifier structures without departing from theteachings of the present invention.

FIG. 5A through FIG. 5F illustrate by way of example embodiments havingdifferent arrangements for the placement of the reference read path andsense amplifiers. In FIG. 5A the reference read path is shown placed persmall memory block (distributed) or per a memory block which iscontrolled by the address decoder (lumped). The sense amplifier is alsoplaced per smaller (sub) memory block (distributed) or per a wholememory block (lumped), as shown in FIG. 5B. The reference read path andthe sense amplifiers can be placed in a distributed form as in FIG. 5Cor a lumped form as depicted in FIG. 5D. The sense amplifier can beshared by multiple reference read paths as shown in FIG. 5E or thereference read path can be shared by multiple sense amplifiers asdepicted in FIG. 5F. It should be recognized that combinations of theabove may be implemented and that variations can be introduced by one ofordinary skill in the art according to the teachings herein withoutdeparting from the present invention.

Another aspect of the invention provides a sensing scheme for the cellstructure which has a read path only from one bit line. The inventivesensing scheme contains a reference read path which can provide areference current to make a signal difference on the bit lines. Thereference read path can provide a different current driving capabilityto eliminate an unnecessary precharge step and to generate a signaldifference on the bit lines. The reference read path can be anystructure to provide a reference current to develop a signal differenceon bit lines.

FIG. 6 depicts a dual-port SRAM variation of the SRAM previouslydescribed. It can be seen from the schematic that the reference line issplit and that two separate sense circuits are provided with the dualdata outputs generated from comparisons against V_(ref1) and V_(ref2)thus providing two separate outputs. It can also be seen in the figurethat the functional blocks of the circuit are configured with differentvoltage thresholds, for example the read sensing blocks are shownincorporating low Vt transistors, which have a lower voltage thresholdthan the transistors of the static memory latch to reduce leakage whileincreasing speed. It should be appreciated that numerous similarvariations of the present invention can be implemented without departingfrom the teachings herein.

The present invention provides new cell structures for SRAM devices andthe like. The structures can incorporate separate functional blocks forwrite paths, read paths and storage which are preferably designed withdifferent threshold voltages to suppress leakage current in the storagepart while improving read speed. The use of separate read and write wordlines is also described for reducing power requirements and facilitatinglow leakage read operations. A form of differential read sensing is alsodescribed in which one terminal of stacked transistors is connected to abit line and another terminal is connected to a virtual source nodewhich is connected to a source transistor to suppress the total leakagecurrent of memory cell block.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more.” All structural and functional equivalents to theelements of the above-described preferred embodiment that are known tothose of ordinary skill in the art are expressly incorporated herein byreference and are intended to be encompassed by the present claims.Moreover, it is not necessary for a device or method to address each andevery problem sought to be solved by the present invention, for it to beencompassed by the present claims. Furthermore, no element, component,or method step in the present disclosure is intended to be dedicated tothe public regardless of whether the element, component, or method stepis explicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 3.5 U.S.C. 112, sixth paragraph,unless the element is expressly recited using the phrase “means for.”

1. A memory device providing static random-access, comprising: a staticmemory cell structure having a plurality of data latches; and aplurality of functional blocks within said cell structure, said blockscomprising at least read, write and storage blocks; wherein the readblocks or write blocks, or both, are configured with a lower thresholdvoltage than the storage functional blocks.
 2. A memory as recited inclaim 1, wherein said read functional blocks have a lower thresholdvoltage than the write and storage functional blocks.
 3. A memory asrecited in claim 1, wherein said read functional blocks have a lowerthreshold voltage than the write functional blocks, and said writefunctional blocks has a lower threshold voltage than the storagefunctional block.
 4. A memory as recited in claim 1, wherein said cellstructure utilizes the same word lines for the read and write paths. 5.A memory as recited in claim 1, wherein said cell structure utilizesseparate word lines for the read and write paths.
 6. A memory as recitedin claim 5, wherein said read word lines activate transistors coupled tobit lines for developing signal differences on the bit lines which aredifferentially sensed for detecting memory storage cell state.
 7. Amemory as recited in claim 5, wherein one group of read path transistorsare connected together with another group of read path transistors to areference read line, virtual node, which is connected to a senseamplifier and a source transistor to suppress leakage current of cellblock.
 8. A memory as recited in claim 7, wherein said source transistorcomprises an NMOS or PMOS transistor.
 9. A memory as recited in claim 7,wherein said reference read path and said sense amplifiers may be placedin a layout configuration within said memory device which is eitherdistributed or lumped.
 10. A memory as recited in claim 7, wherein saidreference read path and said sense amplifiers may be utilized in ashared or dedicated configuration within said memory device.
 11. Amemory as recited in claim 1, wherein said cell structure comprises amulti-port static ram cell structure.
 12. A memory device, having astatic random-access memory configuration, comprising: a plurality ofstatic memory cells formed from data latches; a plurality of word linesfor controlling the read path; a plurality of word lines for controllingthe write path; and a plurality of functional blocks including read,write and storage; wherein at least one of said functional blocks isconfigured to have different threshold conditions that the otherfunctional blocks.
 13. A memory device as recited in claim 12, furthercomprising a virtual node to which read path transistors are connectedbetween different bit lines.
 14. A memory device as recited in claim 13,further comprising a sense amplifier configured for performingdifferential sensing between the bit lines.
 15. A memory device asrecited in claim 13, wherein the connection to said virtual nodesuppresses leakage current of cell blocks.
 16. A method of accessingcells of a static memory, comprising: maintaining data written totransistors of a first voltage threshold level forming a data latch fora static memory cell; applying a read word signals to activate read wordtransistors of a second voltage threshold which is less than firstvoltage threshold; and sensing the output of the read word transistorsin a sense amplifier coupled to a bit line to generate a data bitoutput.
 17. A method as recited in claim 16, wherein said senseamplifier is coupled between two bit lines and the data output isgenerated in response to differential sensing of data output betweensaid two bits lines.
 18. A method as recited in claim 16, furthercomprising writing to said static memory cell in response to activatingtransistors driven by a separate write word line which loads the latchof the static memory cell from bit line data.
 19. A method as recited inclaim 18, wherein said transistors activated by said separate write wordline are configured with a third voltage threshold that is less thansaid transistors of said latch.
 20. A method as recited in claim 19,wherein said third voltage threshold is higher than the voltagethreshold of said transistors activated by said read word line.